High performance digital to analog converters (DACs), such as those utilized in digital video encoder systems, typically must support a digital input resolution of twelve (12) or fourteen (14) bits, and clocks speeds In excess of 100 MHz. One very popular DAC architecture, utilized in such high performance applications, is the segmented current steering DAC. Generally, in a segmented current steering DAC, current elements are partitioned into segments, with at least one of the segments controlled by thermometer-encoded data, such that within that segment, the current elements are equally weighted. Advantageously, thermometer encoding and equally weighted current elements help minimize some types of non-linearities.
On the other hand, segmented current steering DACs require a significant amount of chip area. For example, fifteen (15) thermometer-encoded bits are required to represent four (4) binary bits. Hence, a current steering segment converting four (4) binary bits into an analog signal, after thermometer encoding, requires fifteen (15) current elements.
Additionally, most conventional high resolution DACs require that a much higher unit cell area be utilized during device fabrication to reduce random mismatch, and/or sophisticated cell randomization circuitry to reduce gradient mismatch errors. The result is an increase in device chip area and increased circuit complexity. A few reported DACs use calibration methods which are complex and consume high silicon area.
Given the importance of reducing chip-area in order to fabricate economical and efficient DACs, new techniques are required for performing on-chip calibration to compensate for differences between current steering cells in current steering DACs. These techniques also should not significantly increase the overall area and complexity of the embodying DAC, without adversely impacting performance. In particular, these techniques should be particularly applicable to high-resolution segmented current steering DACs, although not necessarily limited thereto.